1. Field of the Invention
The present invention generally relates to electrical circuit boards and their fabrication. More particularly, this invention relates to a method for providing a planar surface on a circuit board to allow screen printing a polymer thick-film resistor with improved dimensional and electrical tolerances.
2. Description of the Prior Art
Thick-film resistors are employed in hybrid electronic circuits to provide a wide range of resistor values. Such resistors are formed by printing, such as screen printing, a thick-film resistive paste or ink on a substrate, which may be a printed wiring board (PWB), flexible circuit, or a ceramic or silicon substrate. Thick-film inks used in organic printed wire board construction are typically composed of an electrically-conductive material, various additives used to favorably affect the final electrical properties of the resistor, an organic binder and an organic vehicle. After printing, the thick-film ink is typically heated to dry the ink and convert it into a suitable film that adheres to the substrate. If a polymer thick-film (PTF) ink is used, the organic binder is a polymer matrix material and the heating step serves to remove the organic vehicle and cure the polymer matrix material.
The electrical resistance of a thick-film resistor is dependent on the precision with which the resistor is produced, the stability of the resistor material, and the stability of the resistor terminations. The "x" and "z" dimensions (the width and thickness, respectively, of the resistor) of a rectangular PTF resistor are typically determined by a screen printing process, while the "y" dimension (the electrical length of the resistor) is determined by the resistor termination pattern. Conventional screen printing techniques generally employ a template with apertures bearing the positive image of the resistor to be created. The template, referred to as a screening mask, is placed above and in close proximity to the surface of the substrate on which the resistor is to be formed. The mask is then loaded with a PTF resistive ink, and a squeegee blade is drawn across the surface of the mask to press the ink through the apertures and onto the surface of the substrate. Copper terminations are typically formed prior to deposition of the ink by additive plating or electrolytic panel plating with subtractive etching, both of which are capable of achieving a high level of edge definition that enables accurate determination of the electrical length (y) of the resistor.
Compared to many other deposition processes, screen printing is a relatively crude process. As a result, screen-printed PTF resistors are typically limited to dimensions of larger than about one millimeter, with dimensional tolerances generally being larger than about .+-.10% at this lower limit. While the y dimension (electrical length) of a screen-printed PTF resistor can be accurately determined by using appropriate processes to form the terminations, control of the x and z (width and thickness) dimensions of a PTF resistor is fundamentally limited by the relatively coarse mesh of the screen and by ink flow after deposition. Control of resistor dimensions is further complicated by the variability of the surface on which the resistive ink is printed, due in large part to patterned copper interconnects for these resistors having typical thicknesses of about ten to thirty-five micrometers. The interconnect prevents a smooth squeegee action across the surface, resulting in imperfect printing of the screen image and non-uniform deposition of the resistor ink. Consequently, resistance tolerances of less than .+-.20% are difficult to achieve with screen-printed PTF resistors without laser trimming, an operation that is usually cost prohibitive for complex circuits.
From the above, it can be seen that what is needed is a method for forming PTF resistors with more accurate dimensions. Fully additive electroless plating can be used to produce copper interconnect that is substantially coplanar with the dielectric, yielding a planar board surface that enables improved printing precision. However, electroless plating is a very slow and expensive process compared to electrolytic panel plating and subsequent subtractive etching.